Tin and tin-lead plating baths using alkane or alkanol sulfonic acids and their salts in the place of fluoroboric acid and its salts are well known and have been widely put into production in recent years. Corrosiveness of the fluoroboric acid systems and related high maintenance costs have thus been avoided.
Recently, several low to moderate foaming alkane and alkanol sulfonate baths have been patented (see, e.g., U.S. Pat. Nos. 4,871,429; 4,880,507; and 4,923,576). Low foaming characteristics are important for plating baths utilized in modern high speed plating equipment. High current density operation causes evolution of large volumes of gas at the electrodes, causing gross foam buildup. Additionally, pumping and higher operating temperatures associated with high operating speeds can contribute to the foaming problem. Baths can foam over the tank walls, resulting in operational shutdown and exposure to toxic chemicals.
In high speed plating systems, parts are usually carried through the plating tanks on a continuous belt. In many cases, it is desirable for economic or end-use reasons to plate the parts to only a certain height. A head of foam in the plating tank can interfere detrimentally with such a process because the portion of the parts within the foam head are partially plated. It is thus impossible, in a high foaming system, to sharply define plated and unplated regions of the parts.
High current densities and higher operating temperatures associated with continuous plating operations contribute to the loss by evaporation of commonly used brightening agents, typically lower molecular weight aldehydes. Most brightening systems are made up of a low molecular weight aldehyde, such as acetaldehyde (bp. 21.degree. C.), plus in many cases a substituted aromatic aldehyde and/or ketone (see for instance, U.S. Pat. Nos. 4,132,610; 4,139,425; 4,384,930; 4,629,999 and 4,844,780). Even at low to moderate plating speeds, acetaldehyde must be replenished frequently due to its high volatility. At high plating speeds, volatilization of acetaldehyde and other low molecular weight aldehydes is rapid. The resulting high concentration of atmospheric acetaldehyde in the vicinity of the plating equipment presents a serious pollution problem.